Respirators that filter air for breathing are frequently worn by individuals who work in areas where contaminated air is present. The respirators may operate under negative pressure, in which the wearer's lungs provide the power that draws air through the filter (see, for example, U.S. Pat. RE35,062 to Brostrom et al.), or they may operate using positive pressure, in which a fan or other device drives the ambient air through the filter (see, for example U.S. Pat. No. 7,748,381 to Croll et al). A powered air purifying respirator (PAPR) is often desired by users because the wearer does not have to supply the energy needed to force the ambient air through the air filter. The wearer accordingly feels more comfort and may use the saved energy for other tasks.
PAPRs typically have (i) an electric motor and blower unit to force the air through the filter, (ii) a facepiece for delivering the clean air to the user, and (iii) a power source, such as a battery pack, to supply the energy needed to power the device. Known PAPRs have been assembled in a variety of configurations, but two common types are belt pack PAPRs and helmet PAPRs. Belt pack PAPRs typically have the filtering unit worn about the user's waist, whereas helmet PAPRs have the filtering unit contained within the helmet. In both systems, an electrically-powered fan drives or draws the air through the filter cartridges, through the hose, and into the facepiece interior. Because the fan does the work required for air movement through the PAPR system, the user is able to comfortably receive a clean supply of air with little effort.
Helmet-style PAPRs typically use a supported filter bag to filter air before it enters the interior gas space for breathing. The filter bag holders sometimes have only limited or no support through the center of the filter bag because the airflow alone is sufficient to keep the filter layers separated. FIG. 1 shows a filter support used in some helmet-mounted respirators. The filter bag holder 110 is designed to support a flat filter bag in an arcuate form to fit within the crown space of a helmet. The holder 110 is constructed of two members 112 and 114, with the smaller member 114 being held in compression to provide an opening 116 between the two members at one end thereof. Both members 112 and 114 include a plurality of openings 118 and 120, respectively, that are aligned along the length of the holder 110. The filter bag holder 110 is designed primarily to maintain the filter bag in an arcuate shape. An example of a helmet-mounted PAPRs system is disclosed in U.S. Pat. No. 4,280,491 to Berg et al.
Another product that maintains the filtering bag in an arcuate shape is disclosed in U.S. Pat. No. 6,279,570 to Mittelstadt et al. As shown in FIG. 2, this filter support 200 has ribs 210 and 220 that are generally aligned with the longitudinal axis of the device. Some of the support ribs 210 are laterally offset from adjacent ribs 220. FIG. 3 shows how the filtration bag 310 may be placed around the support 200 in a helmet 300.
Another PAPR is described in International Publication WO 2011/126884 to Ausen. In this device the blower is placed within the helmet along with the filter media and a plenum that delivers ambient air to the filter. Air that exits the filter media then passes into another plenum where it is pulled into a blower assembly located centrally within the helmet. After passing through the blower assembly, the filtered air is then delivered to the wearer via a filtered air outlet and a filtered air passageway.
Although these conventional filtering devices have provided good support for the filtering material and have demonstrated good filtration of the air that passes through the device, the devices have not allowed for the controlled flow of air through the filter media. Without such controlled flow, certain portions of the filter media may become expended before others, resulting in an earlier ending of the service life.